1. Field of the Invention
The present invention relates to an optical encoder device for a small-sized motor which optically detects rotation of a motor shaft.
2. Description of the Related Art
In some cases, there arises demand for attaching, to a small-sized motor, a device for detecting the rotational speed and position of the motor. A small-sized motor equipped with an optical encoder device used as such a detection device has been used in applications which require rotation control such as automotive electrical components and OA apparatuses (e.g., a printer).
FIG. 8 illustrates a conventional technique of attaching an optical encoder device to a motor (see Japanese Patent Application Laid-Open (kokai) No. 2003-199297). As shown in FIG. 8, a pair of through holes are formed in a flat side surface (a lower surface in FIG. 8) of a motor so as to allow connection to a printed circuit board. The printed circuit board includes a pair of contact pins adapted to be inserted into the pair of through holes, as well as electronic components such as a photo sensor module and a connector.
After the contact pins are inserted into the through holes by means of pushing and moving the printed circuit board in a direction of an arrow from the lower side in the drawing, a fixing clamp is fitted onto the motor from the upper side in the drawing. That is, the fixing clamp is pushed and moved in a direction of an arrow from the side of the motor opposite the printed circuit board, and pegs of the fixing clamp are inserted into corresponding guide holes formed in the printed circuit board, whereby the printed circuit board is assembled. A code wheel is secured to a shaft end of the motor.
As described above, when the illustrated printed circuit board is employed, a motor having female terminals (through holes) on its side surface is used, and the female terminals and the printed circuit board are connected via the contact pins standing on the printed circuit board. Since the printed circuit board cannot be supported by means of contact pressure of the contact pins applied to the female terminals, an expensive metal fixing clamp is used. However, the complicated shape of the fixing clamp considerably increases cost. Since the positioning between the code wheel and the sensor is realized by a plurality of components cumulatively assembled (such as the motor and the code wheel, the clamp and the motor, the clamp and the board, and the board and the sensor), anxiety arises in relation to positioning accuracy.
In a case where a printed circuit board must be attached to a motor, the motor is typically of a type in which male terminals (motor terminals) project from the end face of an end bell, rather than the side surface of the motor, along the axial direction of the shaft (see Japanese Patent Application Laid-Open (kokai) No. 2006-129692). However, in the case of a motor having a small diameter, such as 20 mm, the male terminals hinder mounting of various components onto the board, which is restricted in dimensions. According to a known technique which can solve such a drawback, a motor of a type in which male terminals project from a side surface (flat surface) of the motor is used, and a printed circuit board is attached to the male terminals. Since the connection between the printed circuit board and the motor is established by means of soldering to the male terminals, the printed circuit board can be fixed to the motor. However, since the printed circuit board is fixed by means of soldering only, the resistance of the printed circuit board to external force is insufficient. Therefore, a thick glass epoxy board which is high in strength but expensive must be used for the printed circuit board. Even in such a case, a resultant structure cannot be said to have a sufficient degree of robustness. The positional relation between the code wheel and the photo sensor module is important. However, the conventional encoder devices do not have a portion for structurally positioning the photo sensor module. Therefore, the conventional encoder devices require external positioning performed by use of a jig or the like, and concerns arise in relation to workability and positioning accuracy.